In our investigations, we ask questions. We try to find a number of sources of data to show, for example, how many hours the crew worked and slept. Part of this is subjective. If a pilot is asked how many hours he slept two nights ago, it won't always be easy for him to remember. We're somewhat limited by this, but we still manage to analyze whether fatigue was a factor.
On April 4th, 2017. See this statement in context.